A rotorcraft is provided with at least one engine that drives at least one main rotor that serves to provide the vehicle with lift and also with propulsion. The rotor comprises a hub that is set into motion by a rotor shaft that is driven by the engine, and a plurality of blades are arranged on the hub.
During their rotary motion, the blades are subjected to a force torsor and they are consequently subjected to centrifugal force together with multiple effects due to flapping, to drag, and to twisting, where twisting is caused in particular by changes in pitch that seek to modify the angle of inclination of the blades relative to the plane of the hub.
In addition, the hub generally includes a plurality of arms, optionally flexible in flapping, with the blades being arranged at the ends of the arms. The forces due in particular to centrifugal force are then transmitted to the non-flexible central zone of the hub.
A first known type of blade has a spar that extends from the blade root along the span of the blade and that is arranged at the leading edge of the blade. For convenience, that type of spar is referred to as a “leading-edge spar”.
That type of spar is effective but is found to be inadequate for modern blades that present complex shapes, e.g. for acoustic reasons.
Document DE 1 406 494 discloses a blade provided with horizontal bores.
A second known type of blade has at least a first substantially horizontal spar that is laid flat respectively on the pressure-side and suction-side faces of the blade. Conventionally, the person skilled in the art tends to refer to such a spar that is laid flat as a “distributed spar” since the first spar is distributed over the cord of the blade on the suction-side and pressure-side faces thereof. The terms “distributed spar” and “flat-laid spar” or indeed “flat-laid distributed spar” as used below all relate to a spar of the same type.
It should be observed that it is possible to provide a second spar of the “leading-edge” spar type that is connected to the first spar, i.e. the distributed spar.
The first spar is then fastened to the hub of the rotorcraft rotor specifically for transmitting centrifugal forces thereto.
The second type of blade is entirely suitable for satisfying the requirements of modern helicopters, particularly if the distributed spar is associated with a leading-edge spar.
Nevertheless, and unfortunately, it is found to be difficult to attach a blade fitted with a distributed spar to a rotor hub.
Document U.S. Pat. No. 3,923,422 discloses a first device for performing such a function. At the blade root, the distributed spar is wound around a vertical fastener bushing that extends parallel to the axis of rotation of the rotor.
However, in order to wind vertically around a vertical fastener bushing, it is necessary for the distributed spar that is laid flat on the suction-side and pressure-side faces of the blade to be twisted. Such twisting through about ninety degrees suffers from the drawback of possibly being destructive insofar as the spar is mainly constituted by unidirectional fibers when making blades out of composite materials.
It is also difficult to lay the unidirectional fibers coming from the twisted section of the distributed spar flat on the suction-side and pressure-side faces of the blade.
The twisting thus leads to poor reproducibility from one blade to another both in terms of shape and also in terms of mechanical and vibration characteristics.
Furthermore, if filler elements need to be arranged in the blade, then they are of complex shape and they vary from one blade to another since the twisting of the spar is not reproduced identically. It then becomes difficult to automate blade fabrication.
It is then possible to envisage winding the spar “horizontally”, in accordance with the second device described in document FR 2 186 380.
Nevertheless, that blade does not have fastener means. It is arranged in an arm of a rotorcraft hub that is of a very special shape, and it is then secured to the arm by means of a horizontal fastener pin that passes through the arm so as to hold the blade spar where it is wound.
Very likely for avoiding excessive flapping of the blade, the arm has top and bottom plates clamping onto the spar of the blade.
Nevertheless, that second device appears not to be particularly suitable for handling forces due to blade drag.
Consequently, the person skilled in the art tends to prefer solutions that make use of vertical fastener bushings, like the first device described above.
Given the specificity of the technical field of rotorcraft blades, it can be understood that documents EP 0 429 353 and EP 0 353 672 describe vanes that are provided with bushings that are vertical, i.e. extending perpendicularly to the plane of rotation of said vanes.